1. Field of the Invention
This invention relates to an image scanning system for two-dimensionally scanning a light beam along a recording sheet material, and more particularly to an improvement in a sub-scanning mechanism of an image scanning system in which the main scanning is conducted using a light deflector and the sub-scanning is accomplished by mechanically moving a recording sheet material.
2. Description of the Prior Art
For recording an image or reading out a recorded image as for example in facsimiles, there has been in wide use an image scanning system in which a light beam is one-dimensionally deflected by a deflector to form a scanning line on a recording sheet material (main scanning) and the recording sheet material is moved in a direction perpendicular to the scanning line (sub-scanning), whereby a plurality of scanning lines arranged in the direction of the sub-scanning in parallel to each other are formed. In the image scanning system, the main scanning must be conducted with a constant period and the recording sheet material must be moved in the sub-scanning direction at a constant speed. When the speed at which the recording sheet material is moved in the sub-scanning direction (sub-scanning speed) varies, the density of the scanning lines fluctuates. The fluctuation in the density of the scanning lines results in fluctuation in the density of the image, thereby lowering the quality of the image. This problem is especially significant when processing images having continuous gradation. See, for example, "Various Problems in the Sub Scanning in a Cylindrical Facsimile" ("National Technical Report" Pages 550 to 558, No. 55, vol. 22, October 1976), or "Visibilty and Correction of Periodic Interference Structures in Line-by-Line Recorded Images" ("Journal of Applied Photographic Engineering" Pages 86 to 92, No. 2, vol. 2, April 1976). Further, our experiments have revealed that when the sub-scanning speed fluctuates by 0.1% in recording an image with a density of 10 to 20 lines/mm, the obtained image exhibits visually detectable density fluctuation.
Conventionally, the sub-scanning mechanism for feeding the recording sheet material at a constant speed with a high accuracy comprises a screw rod which is engaged with a nut fixed to a table for carrying a recording sheet material. By rotating the screw rod at a constant speed, the table is fed at a constant speed in the axial direction of the screw rod. This sub-scanning mechanism is disadvantageous in increasing the processing rate since the table must be returned to the original position after the scanning of one sheet material is finished and the image scanning operation must be interrupted while the table is returned. Further, the sub-scanning mechanism is expensive since the screw rod, the nut and a guide rail for guiding the table must be produced with a very high accuracy and since various complicated mechanisms are required for fixing the recording sheet material on the table, discharging the recording sheet material from the table and locating the recording sheet material in a predetermined position on the table. Since such complicated mechanisms are combined, the recording sheet material is apt to be positioned in the wrong position and the sub-scanning mechanism frequently gets jammed with the recording sheet material.